1. Field of the Invention
The present invention generally relates to a constant velocity universal joint and a method of manufacturing the same. More particularly, the present invention relates to the constant velocity universal joint of a type capable of withstanding rolling fatigue for a prolonged period of time and having a high resistance to cracking and, also, a resistance to secular change in dimension, and the method of manufacturing such constant velocity universal joint.
2. Description of the Prior Art
The constant velocity universal joint is well known in the art and is so designed to connect drive and driven shafts together there through so that even though the drive and driven shafts lie at an angle relative to each other, a rotatory drive can be transmitted from the drive shaft to the driven shaft
The constant velocity universal joint generally comprises an outer coupling member, an inner coupling member and a rollable drive-transmitting member interposed between the outer and inner coupling members. Since in the constant velocity universal joint, the rotatory drive is transmitted from one of the outer and inner coupling members to the other of the outer and inner coupling member while the angle between the drive and driven shafts is accommodated by a rolling motion of the drive transmitting member, the drive transmitting member and the outer and inner coupling members are placed under severe conditions with respect to the rolling fatigue. Because of this, heat treatment is generally applied to the inner and outer coupling members and the drive-transmitting member so that they can have a prolonged lifetime against the rolling fatigue. In this connection, the need has arisen to improve the heat treatment method to enhance the rolling fatigue life.
On the other hand, the constant velocity universal joint is currently available in some types, including a tripod type, an improved version of the tripod type such as disclosed in the Japanese Laid-open Patent Publication No. 2000-320563, and a ball joint type.
In a rolling bearing which is a mechanical component of a type utilizing rolling elements as is the case with the constant velocity universal joint, as a method of heat treatment effective to increase the lifetime of the rolling bearing against the rolling fatigue, the Japanese Laid-open Patent Publication No. 8-4774 discloses a heat treating method in which the atmosphere under which heat treatment is carried out contains an ammonium gas in addition to the RX gas so that a surface region of a component part of the rolling bearing can have a carbonitrided layer. The use of this known carbonitriding treatment is effective to harden the surface region of the component part, causing a residue austenite structure to grow in the microstructure to thereby increase the rolling fatigue life.
However, since the constant velocity universal joint operates to transmit the rotatory drive from one of the inner and outer coupling members to the other of the inner and outer coupling members while the angle between the drive and driven shafts is accommodated by the rolling motion of the drive transmitting member, a relatively large load tends to act on contact portions of the drive transmitting member, with the bearing pressure at portions of contact between the inner and outer coupling members and the drive transmitting member increased consequently. In addition, in the constant velocity universal joint employed in the modern automotive vehicles, the need has arisen to reduce the size and weight of the constant velocity universal joint and, therefore, improvement in rolling fatigue lifetime is pressing.
Since the carbonitriding treatment is a diffusion process in which carbon and nitrogen are diffused, the material to be treated must be retained at an elevated temperature for a substantial length of time. In view of this, it is difficult to increase the resistance to cracking because the structure tends to become coarse. Also, a problem is arisen in that secular change in dimension tends to increase as a result of increase of the residue austenite.
On the other hand, to secure a prolonged lifetime against the rolling fatigue, increase the resistance to cracking and to inhibit increase of the secular change in dimension, it may be possible to achieve those objectives by employing an alloy instead of steel. However, with the design of an alloy, a problem such as an increase of material cost would arise.
Considering that a high load and an elevated temperature in the environment of use are increasingly imposed on component parts of the constant velocity universal joint, future's component parts of constant velocity universal joints are required to have such characteristics as to withstand against the high load and the elevated temperature. Accordingly, the need has arisen to develop and manufacture those component parts having a high strength, a prolonged lifetime against the rolling fatigue, a high resistance to cracking, and a high stability against secular dimensional change.